The present invention relates to auto chroma circuits, and particularly to a chroma filter circuit which detects and maintains an average DC value of chroma amplitude on every video line per frame, averaged over many frames.
Video production recorders such as those commonly known as helical scan recorders, utilize an equalizer circuit whose major function is to equalize radio frequency (RF) signals produced by the video and sync play heads, and then route the signals to a demodulator circuit in the recorder. The equalizing function removes errors that are generated by variations in head-to-tape contact, the position of the head with respect to a recorded track as in the case of a dithered playback head, scratches on the tape, etc. The error signals are reflected as a decrease in the level of the RF retrieved from the tape.
In addition, helical video tape recorders utilize an auto chroma system to measure the average DC value of the amplitude of a color burst signal. The signal is sent to the equalizer circuit which produces an average chroma amplitude signal via a servo loop to maintain the video bandwidth, i.e., to maintain as flat a frequency response as possible across a full field or frame of the video signal. Such chroma circuits typically detect the amplitude of a color burst at the beginning of a frame, and thereafter maintain the same error correction signal throughout the entire frame interval.
However, since the signal recovered from tape is very noisy due to the previously mentioned head-to-tape spacing variations, dropouts, etc., the average error signal detected over an entire frame interval is completely hidden in the noise. It follows that many variations in the signal level may occur during one frame that are not detectable by typical auto chroma circuits. That is, present auto chroma circuits fail to correct for small amplitude variations in the equalizer error signal off tape which occur in one or two lines of video.
The present invention overcomes the shortcomings of present auto chroma circuits by detecting and maintaining an average DC value of chroma amplitude on every video line during a frame interval, averaged over many frames. The resulting additional correction signal is added to the DC correction signal of the auto chroma filter systems.
To this end, the amplitude of every successive color burst is sampled and is supplied to a comparator along with the analog output of the auto chroma filter circuit. A random access memory stores the digital value of the burst amplitude of every horizontal line in a television frame. A digital value of color burst from a line is fetched from memory and is loaded into an up/down data counter, and also is fed to a digital-to-analog (D/A) converter to provide the analog output to the equalizer as well as to the comparator. The comparator compares the incoming input data with the previously averaged analog output from the D/A converter, and directs the up/down counter to increment or decrement its contents toward the input data value. The magnitude of the counter increment or decrement is determined by a time constant counter oscillator which selects the number of clock pulses that are used per burst sample that are supplied to the updating components. The updated value generated in the up/down counter is loaded back into memory at the same location from which the previous value was fetched. The newly stored data is then fetched again in the next frame.
Accordingly, it is an object of the present invention to provide an average DC value of chroma amplitude on every video line per frame averaged over many frames.
It is another object to compare, line by line, a color burst value stored in memory with an incoming color burst value in order to update the burst value prior to storage back in memory.
Another object is to provide the average DC value of color burst for every horizontal line in a video frame, by incrementing or decrementing a counter to cause an old value to approach a new incoming data value.